1. Field of the Invention
This invention relates to a carrier system for positioning sensors adjacent a wire rope for measuring longitudinal movement of the wire rope to determine depth in a well of equipment which is carried by wire rope, while avoiding errors caused by lateral movement of the wire rope.
2. The Prior Art
The accurate measurement of equipment depth in a well is of major importance, and various arrangements have been employed to effect such measurement.
Some arrangements sense the rotation of the crownwheel on the derrick and thereby provide an indication of the longitudinal movement of the wire rope which actuates the kelly and the drill stem. Such arrangements require installation and maintenance work at the top of the derrick which may be dangerous, especially in bad weather or on floating drill rigs. Such arrangements suffer the disadvantage of inaccuracies introduced by slippage of the wire rope on the crownwheel or by slippage of a mechanical wheel employed to sense movement of the crownwheel.
Another arrangement measures the rotation of the drum of the drawworks and the layers of the wire rope that are spooled on the drum to measure longitudinal movement of the wire rope. Such an arrangement is rather complex, and it is not convenient for use in well logging because well logging techniques require self contained apparatus that can be readily applied to a drilling rig and that can be moved from one drilling rig to another.
Still another arrangement measures the incremental movement of the kelly by apparatus mounted at the top of the kelly. Such an arrangement is subject to considerable vibration and shock due to the movements of the kelly. It is difficult to service, particularly during the drilling operation.